1. Field of the Invention
Embodiments of the present invention relate to electric tools having electric motors as drive sources, and in particular to rotary electric tools, such as disk grinders and screwdrivers.
2. Description of the Related Art
In an electric tool having an electric motor as a drive source, the rotational power of the electric motor may be output to an end tool or a working tool, such as a grinding wheel or a driver bit, via a reduction gear train. In this kind of electric tool, when the electric motor starts, for example through the operation of a switch, it may be possible to generate an impact called a “start shock” that is applied to a torque transmission system or the working tool.
In order to solve the problem of this start shock generation, JP-A-2002-264031 and JP-A-2010-179436 have proposed start shock attenuation structures. According to these attenuation structures, a C-shaped torque transmission member resiliently deformable in a radial direction may be interposed between two rotary elements in a torque transmission system. Therefore, when the rotational torque is transmitted from one to the other of the two rotary elements, the torque transmission member may resiliently deform in a radial direction (i.e., a diameter enlarging direction) according to a load applied to the rotary element on the driven side. In this way, the start shock can be attenuated. As a result, it is possible to improve the durability and the feeling in use of the electric tool.
However, in the case of the above known attenuation structures, if they are applied to a large-size electric tool having a large electric motor output, it may be possible that the C-shaped torque transmission member can be instantaneously opened to be enlarged, leading to an insufficient shock attenuation effect. In light of this, it may lead to an increase in the thickness of the torque transmission member. However, in this case, a possibility of metallic fatigue of the torque transmission member due to repeated application of the load may lead to increased damages, such as cracking and breakage. As the size of the electric tool increases, the possibility of causing damages may be increased. Damages to the torque transmission member may reduce the durability of the electric tool.
Therefore, there has been a need in the art for a technique of attenuating a start shock without leading to damages to a torque transmission member.